Esters are among the most important and abundant functional groups in chemistry and they are widely found in food, pharmaceutical, flavor, and fine and bulk chemical industries. There are a number of classical methods, e.g. reaction with carboxylic acid derivatives, carbonylation and the Tischenko reaction, which can be used to prepare ester compounds. The coupling of aldehydes with alcohols and coupling of two alcohols in the presence of external oxidants can also form esters. An alternative approach is the dehydrogenative coupling of two alcohols or aldehydes with alcohols with the release of H2.
Given the abundance of aldehydes in the marketplace, the dehydrogenative cross-coupling of aldehydes with alcohols could provide an attractive way to directly convert the compound into an ester. However, a big issue facing such cross-coupling reactions is that the metal hydride intermediate, expected to be formed during the dehydrogenation reaction, can readily reduce the aldehydes to alcohols, instead of undergoing desired protonation to form H2.
Such reductions of aldehydes to alcohols also lead to the formation of undesired homoesters. Therefore, these aldehyde-alcohol dehydrogenative coupling reactions often suffer from poor selectivity and as a result downstream separation of the side-products from the desired ester becomes energy-intensive and costly.
A need exists for a thermally stable catalyst which bypasses the formation of alcohols and homoesters and mediates the exclusive formation (selectivity up to >99.9%) of methyl esters between different types of aldehydes and MeOH under mild conditions (<100° C.).
The present invention addresses this need as well as others, which will become apparent from the following description and the appended claims.